Frequency-doubled diode lasers have been demonstrated. One example of such a device is the laser apparatus described in U.S. Pat. No. 4,951,293 to Yamamoto et al. In that patent, a semiconductor laser and an optical nonlinear device are both located on a submount, with the laser's active layer and the nonlinear device's surface waveguide facing the submount, so that a fundamental light wave from the laser can be directly applied to the nonlinear device and doubled in frequency, thereby providing a visible light output from the nonlinear device.
Second harmonic generators are well known nonlinear optical devices, and are commercially available. For example, U.S. Pat. Nos. 4,953,931 and 4,953,943 to Miyazaka et al. describe devices having a LiNbO.sub.3 thin film waveguide layer formed on a LiTaO.sub.3 substrate, which materials are formulated to have specified refractive indices at the fundamental and second harmonic wavelengths satisfying a desired relationship for efficient second-harmonic generation and waveguiding. U.S. Pat. No. 4,973,117 to Yamada describes a device that has a ridge type optical waveguide formed on a substrate of nonlinear optical material so as to generate a second-harmonic wave by Cerenkov radiation. This ridge type waveguide includes a first waveguide passage for confining a fundamental wave and converting it into the second-harmonic wave and a second waveguide passage for confining the second-harmonic wave and propagating it toward an end face for emission thereof. The first waveguide passage, which is formed so as to be in contact on at least one of the lateral side surfaces of the ridge-shaped second waveguide passage, has a width dimension that is smaller than the cutoff thickness for propagation of the second-harmonic wave so as to leak this radiation into the second waveguide passage.
In U.S. Pat. No. 5,022,729, Tamada et al. describe an optical waveguide and second-harmonic generator having a Ta.sub.2 O.sub.5 TiO.sub.2 -system amorphous thin film optical waveguide formed on a substrate for confining and guiding lightwaves received thereby. The substrate may be a nonlinear optical crystal material, so that a second-harmonic wave can be obtained from the fundamental wave received by the waveguide and then radiated to the substrate side. Generation of visible (0.4 .mu.m) second-harmonic wave light from the near infrared (0.8 .mu.m) fundamental wave light of a semiconductor laser can be made more efficient by forming a periodically-poled region of selected period and depth in the nonlinear material substrate.
In U.S. Pat. No. 5,036,220, Byer et al. describe a nonlinear optical converter having a source of electromagnetic radiation of a frequency different than the desired optical frequency, a solid state material body provided with a waveguide, and means for directing the electromagnetic radiation from the source into the waveguide. The waveguide is characterized by being provided with a plurality of successive regions of differing compositions, such that successive regions form domains of alternating dominant electrical polarization states. The electrical polarization states are aligned transverse to the path of the guided radiation. The resulting waveguide converts the frequency of the radiation received from the source to a desired optical frequency for the output radiation.
By coupling single-mode diode lasers to periodically-poled waveguides, output powers for the frequency-doubled laser light in excess of 0.5 mW cw have been demonstrated. The output power of frequency-doubled diode lasers is limited by the stability of the diode laser's output wavelength, which must be matched to the peak frequency-conversion wavelength of the periodically-poled waveguide for maximum conversion efficiency, and is also limited by the coupling efficiency of the diode laser light to the periodically-poled waveguides.
It is an object of the invention to provide higher power, frequency-doubled, diode lasers.